Most portable
products with displays that use white light emitting diode (WLED)
backlights also need auxiliary LED lighting. Two ICs are generally
needed: an inductive boost to obtain maximum efficiency (>80%)
for the backlight LEDs; and a charge pump to allow independent control
of each auxiliary LED. In addition, each IC requires a programmable
current sink for brightness control or color blending, so the cost
and complexity can increase quickly. This design tip shows how a single
programmable LED driver can be combined with a low-cost boost converter
to achieve a flexible, high-efficiency, easy-to-program solution.
Figure 1 shows an implementation using the ADP1612
(see Figure 2) boost converter and the ADP8860
(see Figure 3) parallel LED driver.

In
this application, FB of the ADP1612 boost converter is connected to
D2, one of the current sinks on the ADP8860 LED driver. The 5-V Zener
diode protects that current sink in case of a fault or rapid shutdown.
The OVP Zener diode protects the output capacitor, COUT,
and the ADP1612 in case of an open-circuit fault in one of the backlight
LEDs.

When
current sink D2 is off, the voltage on FB is pulled to VIN,
shutting down the ADP1612. When D2 turns on, the voltage on FB is
pulled low, and the boost starts switching. The ADP1612 regulates
the output voltage to provide 1.2 V on FB and thus on D2. This is
enough to allow accurate current regulation. As the current through
D2's current sink changes, the ADP1612 automatically scales the output
voltage to deliver exactly enough voltage to power the LEDs and the
current sink. The ADP8860 has independent control of each sink, so
the same programming used for the auxiliary LEDs can also be applied
to the backlight LEDs.

Step-Up
DC-to-DC Switching Converters Operate at 650 kHz/1300 kHz
The ADP1612
and ADP1613 step-up converters are capable of supplying over 150 mA
at voltages as high as 20 V, while operating with a single 1.8-V to
5.5-V supply or single 2.5-V to 5.5-V supply, respectively. Integrating
a 1.4-A/2.0-A, 0.13-Ω
power switch with a current-mode, pulse-width modulated regulator,
their output varies less than 1% with changes in input voltage, load
current, and temperature. The operating frequency is pin-selectable
and can be optimized for high efficiency or minimum external component
size: at 650 kHz they provide 90% efficiency; at 1.3 MHz their circuit
implementation occupies the smallest space, making them ideal for
space-constrained environments in portable devices and liquid-crystal
displays. The adjustable soft-start circuit prevents inrush currentsensuring
safe, predictable start-up conditions. The ADP1612/ADP1613 consume
2.2 mA in the switching state, 700 µA
in the nonswitching state, and 10 nA in shutdown mode. Available
in an 8-lead MSOP package, they are specified from 40°C
to +85°C and priced at $1.50/$1.20 in 1000s.

Figure
2. ADP1612/ADP1613 block diagram.

7-Channel
Smart LED Driver Includes Charge Pump, I2C
Interface
The ADP8860 smart LED driverwhich combines a programmable
charge-pump driver with automatic phototransistor controlchanges
current density according to ambient light conditions, eliminating
the need for a processor and allowing significant power savings in
mobile displays. As many as six LEDs can be independently driven at
up to 30 mA; a seventh LED can be driven at up to 60 mA. Light intensity
thresholds, min/max LED current, and fade in/out times are all programmable
via the I2C interface. The two-capacitor charge-pump can
source 240 mA. Automatic gain selection of 1×, 1.5×, or 2× maximizes its efficiency.
Safety features include soft start, undervoltage lockout, and short-circuit-,
overvoltage-, and overtemperature protection. Operating with a single
2.5-V to 5.5-V supply, the ADP8860 consumes 4.5 mA in switching mode
and 0.3 μA
in standby mode. Available in 20-lead LFCSP and 20-ball WLCSP packages,
it is specified from 40°C to +85°C and priced from $1.36
in 1000s.

Figure
3. ADP8860 functional block diagram.

Author

Jon
Kraft [jon.kraft@analog.com]
joined Analog Devices in 2007 and works as an applications engineer
at the Power Management Design Center in Longmont, Colorado. He
holds a BSEE from Rose-Hulman Institute of Technology and an MSEE
from Arizona State University; he has been awarded three patents.(return to top)

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